Defragmentation and cleanup on traditional hard drives take longer when they're full for different reasons including the increased amount of data being managed and in many cases empty space being further away from the data being moved which will slow down moving of data to clear and consolidate free space. On a solid state drive the bigger issue is that there is a minimum size block of data that can be written in one operation. Having empty space available means that the drive can write data into that empty space without having to do garbage collection first.